There's a lot of advice of buying into an unmanaged S&P 500 fund and holding it. But since it's unmanaged no financial advisor is going to advise you to do that because there's nothing in there for them. So, I am a Canadian individual but also I have a small company. How could I and also how could my company buy into these then?
As user quid states in his answer, all you need to do is open an account with a stock broker in order to gain access to the world's stock markets. If you are currently banking with one of the six big bank, then they will offer stockbroking services. You can shop around for the best commission rates. If you wish to manage your own investments, then you will open a "self-directed" account. You can shelter your investments from all taxation by opening a TFSA account with your stock broker. Currently, you can add $5,500 per year to your TFSA. Unused allowances from previous years can still be used. Thus, if you have not yet made any TFSA contributions, you can add upto $46,500 to your TFSA and enjoy the benefits of tax free investing.
Investing in what you are calling "unmanaged index funds" means investing in ETFs (Exchange Traded Funds).
Once you have opened your account you can invest in any ETFs traded on the stock markets accessible through your stock broker. Buying shares on foreign markets may carry higher commission rates, but for the US markets commissions are generally the same as they are for Canadian markets. However, in the case of buying foreign shares you will carry the extra cost and risk of selling Canadian dollars and buying foreign currency. There are also issues to do with foreign withholding taxes when you trade foreign shares directly. In the case of the US, you will also need to register with the US tax authorities. Foreign withholding taxes payable are generally treated as a tax credit with respect to Canadian taxation, so you will not be double taxed.
In today's market, for most investors there is generally no need to invest directly in foreign market indices since you can do so indirectly on the Toronto stock market. The large Canadian ETF providers offer a wide range of US, European, Asian, and Global ETFs as well as Canadian ETFs. For example, you can track all of the major US indices by trading in Toronto in Canadian dollars. The S&P500, the Dow Jones, and the NASDAQ100 are offered in both "currency hedged" and "unhedged" forms. In addition, there are ETFs on the total US Market, US Small Caps, US sectors such as banks, and more exotic ETFs such as those offering "covered call" strategies and "put write" strategies.
Loads of financial advisors advise holding index funds they may advise other things as well, but low fee index funds are a staple portfolio item. I can't speak to the particulars of Canada, but in the US you would just open a brokerage account (or IRA or SEP IRA in the case of a small business owner) and buy a low cost S&P index ETF or low/no fee/commission S&P index mutual fund. There's no magic to it.
Some examples in no particular order are, Vanguard's VOO, Schwab's SWPPX, and iShares' IVV. There are also Canadian index equities like Vanguard's VCN and iShare's XIC.